At least 18 people have been killed in a gun attack on an intercity bus that was on its way from the Syrian city of Raqqa to the capital Damascus, a Syrian anti-Assad coalition said Tuesday.

According to a statement issued by the Syrian Revolution General Commission, the bus was attacked in Haresta region. All the bus passengers were from Raqqa, which is under control of the Islamic State of Iraq and the Levant militants, the statement added.

The ISIL terrorist group has captured large swathes of land in Iraq and Syria, declaring what it calls a cross-border Islamic caliphate, killing thousands of people and displacing millions more in the process.

The opposition forces in March 2011 began protests to topple President Bashar Al-Assad, a struggle that turned into a civil war in July of that year after regime forces violently suppressed protests.

More than 190,000 people have been killed in Syria since the conflict began, according to UN figures published last August.